The present invention is directed to a Raman amplifier system for amplifying optical signals that are transmitted via a transmission medium.
In present and future optical transmission systems, particularly with respect to transmission systems operating according to the WDM principle (Wavelength Division Multiplexing), a pump wave inserts optical pump signals into an optical standard single-mode fiber in order to provide the necessary optical pump power for optical amplifiers preceding the launching location; for example, erbium amplifiers. Furthermore, such optical pump signals are used for directly amplifying optical signals to be transmitted, whereby the optical amplification caused by the optical pump signals is based on the Raman effect. For example, the Raman effect (“Stimulated Raman Scattering”) is described in “Nonlinear Fiber Optics” by Govind P. Agrawal, Academic Press, 1995, see pages 316 to 322.
The Raman preamplifier is known as a specific embodiment for the use of the Raman effect for optical amplification problems; for example, see “Raman limited, truly unrepeated transmission ad 2.5 Gbit/s over 453 km with +30 dBm launched signal power”, E. Brandon, J.-P. Blondel, pages 563 to 564, ECOC98, 20-24. September 1998, Madrid, Spain or see Govind P. Agrawal, “Nonlinear Fiber Optics”, Academic Press, 1995, pages 356 to 359.
A powerful optical pump signal, immediately before the optical reception device of an optical transmission path, is launched into the optical fiber, whereby the optical pump signal, contrary to the optical data signal, expands in the optical transmission medium or, respectively, in the optical fiber. On the basis of the Raman effect, one Stokes' wave or, respectively, a number of Stokes' waves is or, respectively, are generated in the optical fiber for this powerful optical pump signal, whereby the Stokes' waves are dependent on the wavelength of the pump signal and which amplify different optical signal waves or, respectively, optical signals having different wavelengths in different types of fibers. Given previously known Raman preamplifiers, optical pump signals having wavelengths of approximately 1450 to 1460 nm normally are used in order to effectively preamplify optical signals or, respectively, data signals having wavelengths of 1550 to 1560 nm, whereby the first Stokes' wave is used for preamplifying the first optical signal. As a result, the transmission length of an optical transmission path, which can be bridged in a regeneration-free manner, can be significantly extended, so that an enormous economical advantage is obtained given the realization of an optical transmission path with the assistance of an optical deep-sea cable. An even greater overall transmission length of the optical transmission path can be achieved the further remote from the launching location of the optical pump signal an optimally high pump capacity or, respectively, Raman preamplification can be generated by the Raman effect, whereby the overall transmission length can be bridged in a regeneration-free manner.
The maximum Raman amplification is limited by the loss of reflection of the Rayleigh scattering in the transmission fiber. The Rayleigh scattering is caused in optical standard single-mode fibers by density fluctuations as a result of fiber impurities which accidentally arise during the production of optical standard single-mode fibers. The optical signal to be transmitted is scattered into different directions as a result of the arising local modifications of the refractive index caused by such density fluctuations. Given a Raman amplification that is too high, i.e. (a Raman pump capacity that is too high), the “Amplified Spontaneous Emission” (ASE) (i.e., the optical noise added to the optical signal or, respectively, data signal, of the optical Raman amplifier) is increased such that an independent oscillation of the Raman signal is stimulated in the transmission fiber, whereby this effect is amplified significantly greater, with respect to a Raman amplification, than the Rayleigh scattering reflex is additionally amplified.
Therefore, the maximum optical Raman pump capacity launched into the optical fiber is kept correspondingly low with respect to existing optical transmission systems or, respectively, Raman amplifier arrangements, in order to prevent such oscillation. Furthermore, the effective noise factor of the Raman amplifier arrangement, which can be obtained, is limited given the use of Raman pump capacities of less than 1 Watt, see Govind P. Agrawal, “Nonlinear Fiber Optics”, Academic Press, pages 477-480, whereby the optical signal-to-noise ratio (OSNR) of the optical data signal is additionally reduced as a result.
Furthermore, the publication “Ultra Low Nonlinearity Low Loss Pure Silica Core Fiber for Long Haul WDM-Transmission” by T. Kato et al., Electronic Letters, vol. 35, no. 19, p. 1615-1617, September 1999 discloses optical fibers having a fiber cross-section of more than 110 μm2 and a low damping constant of 0.17 dB/km given a signal wavelength of 1550 nm, whereby the optical fibers have a nonlinearity coefficient which is reduced by 30% compared to traditional optical standard single-mode fibers and which therefore enable an almost distortionfree transmission of optical signals via distances of a few hundred kilometers.
An object of the present invention is to optimize the amplification of optical signals by using the Raman effect.